Golf ball

ABSTRACT

The present invention provides a golf ball having excellent flight performance. The present invention relates to a golf ball comprising a core and a cover covering the core, wherein the cover has many fine open voids formed by foaming on the surface thereof.

FIELD OF THE INVENTION

[0001] The present invention relates to a golf ball. More particularly, it relates to a golf ball having excellent flight performance.

BACKGROUND OF THE INVENTION

[0002] A golf ball has many depressions called “dimples” on the surface. The dimples have function to disturb airflow around the golf ball on the fly and to facilitate turbulent transition at boundary layer so as to give rise to turbulent separation, which is called “dimple effect”. In golf ball having aerodynamically excellent dimples, the facilitation of turbulent transition sifts the separation point of air from the golf ball to backward and reduces a drag coefficient. In addition, the facilitation of turbulent transition increases a difference of the separation point between an upper side and a lower side of the golf ball, caused by backspin of the golf ball, and enhances lifting power applied on the golf ball. Therefore, the flight performance of the golf ball improved by the factors.

[0003] Recently, since it has been required to provide golf balls having further excellent flight performance, the technical effects of improving the flight performance were not sufficiently accomplished only by the conventional dimples. Therefore, a method of forming micro-dimples, which are smaller than the dimples, by roughening the surface of the golf ball by blasting treatment, in addition to the dimples, has been proposed (in Japanese Patent Kokai Publication No. 369896/2002).

[0004] In Japanese Patent Kokai Publication No. 369896/2002, golf ball having many dimples on the surface thereof, which micro-dimples having a diameter of not more than half of dimple diameter are formed inside or outside of the dimple, or over the entire surface of the golf ball, by blasting treatment, is disclosed. However, in the golf ball described above, since open cells are formed on the surface of the cover by blasting treatment, it is difficult to uniformly form the open cells over the entire surface thereof, and it is problem that the trajectory height and flight distance vary depending on the state of the golf ball (lie and direction of the golf ball) at the time hitting. In addition, the directivity of the hit golf ball is poor. Moreover, since the surface of the golf ball after forming dimples are treated by blasting treatment, the dimple edge is not sharp, which reduces the original aerodynamical effects accomplished by the original dimples. In addition, since the blasting treatment is conducted after molding the cover, the number of the manufacturing process is increased, and it is problem that the productivity is degraded, or the production cost is increased.

[0005] In order to solve the problem, it is considered to uniformly form cells by using a foamable material for the cover (described in Japanese Patent Kokai Publication No. 92372/1983). In Japanese Patent Kokai Publication No. 92372/1983, a golf ball comprising a spherically-shaped solid core of resilient material, a first layer of hard, high flexural modulus ionomer resin molded on the core, and a cover layer of soft, low flexural modulus ionomer resin molded over the first layer of ionomer resin on the core, and having the total weight of the golf ball not exceeding 1.620 ounces, is disclosed. As the cover layer material, foamable material is described.

[0006] However, since non-foamed layer called as a skin layer is formed on the surface of the molded article when molding the foamable material, it is problem that open cells are not formed on the surface.

OBJECTS OF THE INVENTION

[0007] A main object of the present invention is to provide a golf ball having excellent flight performance.

[0008] According to the present invention, the object described above has been accomplished by using a cover having many fine open cells formed by foaming on the surface thereof, thereby providing a golf ball having excellent flight performance.

[0009] This object as well as other objects and advantages of the present invention will become apparent to those skilled in the art from the following description with reference to the accompanying drawings.

BRIEF EXPLANATION OF DRAWINGS

[0010] The present invention will become more fully understood from the detailed description given hereinbelow and the accomplishing drawings which are given by way of illustrating only, and thus are not limitative of the present invention, and wherein:

[0011]FIG. 1 is a schematic cross section illustrating one embodiment of the open cell in the golf ball of the present invention.

SUMMARY OF THE INVENTION

[0012] The present invention provides a golf ball comprising a core and a cover covering the core, wherein the cover has many fine open cells formed by foaming on the surface thereof.

[0013] The golf ball 3 of the present invention has many fine open cells 1 on the surface thereof in addition of the dimples 2. It is possible to uniformly form open cells over the entire surface of the golf ball by using foamed cells obtained by foaming the cover as such open cells. Thereby it is possible to restrain the dispersion of the flight distance and poor directivity, which are problems in the open cells formed by blasting treatment described in Japanese Patent Kokai Publication No. 369896/2002.

[0014] The golf ball of the present invention may be produced by grinding and removing the skin layer formed on the surface of the foamed material obtained by the above conventional method. Moreover the golf ball of the present invention may be produced by foaming the cover to the surface thereof to directly form the open cells on the surface of the golf ball, for example, by adjusting the molding condition and solidifying time, or coating a releasing agent on the mold, when solidifying a foamable cover composition containing foaming agent with foaming.

[0015] In order to put the present invention into a more suitable practical application, it is preferable that the open cell have a depth of 0.05 to 0.5 mm, an opening area of 0.005 to 5.0 mm² and a number of 200 to 10,000 per 1 cm².

[0016] Moreover, the present invention provides a method of making a golf ball comprising a core and a cover covering the core comprising the steps of

[0017] (a) forming the core,

[0018] (b) providing a foamable cover composition, and

[0019] (c) covering the cover formed from the cover composition on the core to form the golf ball, wherein the cover has many fine open cells formed by foaming on the surface thereof.

[0020] In addition, the present invention provides a method of making a golf ball comprising a core and a cover covering the core comprising the steps of

[0021] (a) forming the core,

[0022] (b) providing a foamable cover composition,

[0023] (c) covering the cover formed from the cover composition on the core to form the golf ball, and

[0024] (d) removing the surface of the cover to form many fine open cells on the surface of the golf ball, wherein the cover has many fine open cells formed by foaming on the surface thereof.

DETAILED DESCRIPTION OF THE INVENTION

[0025] The golf ball of the present invention will be explained with reference to the accompanying drawing in detail. FIG. 1 is a schematic cross section illustrating one embodiment of the open cell in the golf ball of the present invention. As shown in FIG. 1, the cover has many fine open cells 1 formed by foaming on the surface of the cover in addition to many dimples 2 in the golf ball 3 of the present invention. The golf ball of the present invention comprises a core and a cover formed on the core. The core may be the same one that has been conventionally used for solid golf, and may be obtained by mixing a rubber composition using a proper mixer, such as a mixing roll, and then vulcanizing and press-molding under applied heat the rubber composition in a mold into a spherical form. The rubber composition comprises

[0026] 10 to 0.60 parts by weight of a vulcanizing agent (crosslinking agent), for example, αβ-unsaturated carboxylic acid (such as acrylic acid, methacrylic acid, etc.) or mono or divalent metal salts, such as zinc or magnesium salts thereof, or a functional monomer such as trimethylolpropane trimethacrylate, or a combination thereof;

[0027] 0.5 to 5 parts by weight of co-crosslinking initiator such as organic peroxides;

[0028] 10 to 30 parts by weight of filler such as zinc oxide, barium sulfate and the like; and

[0029] optionally organic sulfide compound, antioxidant and the like, based on 100 parts by weight of a base rubber such as cis-1,4-polybutadiene rubber. The vulcanization may be conducted, for example, by press molding in a mold at 130 to 240° C. and 2.9 to 11.8 MPa for 15 to 60 minutes. It is preferable for the surface of the resulting core to be buffed to improve the adhesion to the cover layer formed on the core. However, such core is given by way of illustrative examples only, and the invention shall not be limited thereto.

[0030] In the golf ball of the present invention, it is desired that the core have a diameter of 36.7 to 41.5 mm, preferably 37.3 to 40.9 mm, more preferably 38.1 to 40.5 mm. When the diameter of the core is smaller than 36.7 mm, the cover is thick, and the rebound characteristics are degraded. On the other hand, when the diameter is larger than. 41.5 mm, the thickness of the cover is too thin, and the technical effects accomplished by the presence of the cover are not sufficiently obtained. In addition, it is difficult to mold it.

[0031] In the golf ball of the present invention, the cover is then formed on the core. In the golf ball of the present invention, it is required for the cover to have many fine open cells formed by foaming on the surface thereof. In the golf ball of the present invention, the cover may have single-layered structure or multi-layered structure, which has two or more layers, as long as the cover has many fine open cells formed by foaming on the surface thereof. When the cover has multi-layered structure, the entire cover may have many fine open cells formed by foaming, but the outmost layer thereof preferably has many fine open cells formed by foaming.

[0032] In the golf ball of the present invention, foamed cover is molded by press molding, injection molding and the like, and the cover is foamed by adjusting the molding temperature. The cells formed by foaming may be formed on the surface of the cover by optimizing the molding condition, or, when the non-foamed skin layer is formed on the surface, the open cells may be formed on the surface of the cover by removing the surface in a depth of about 0.01 to 1.0 mm by barrel grinding treatment, abrasive stone grinding treatment and the like.

[0033] In the golf ball of the present invention, it is desired to adjust the depth, open area and number of the open cell to the following ranges in order to maintain the effect of facilitating turbulent transition by the open cells and aerodynamic effect by the dimple. It is desired for the open cell to have a depth of 0.05 to 0.5 mm, preferably 0.07 to 0.2 mm, more preferably 0.08 to 0.15 mm. When the depth is smaller than 0.05 mm, the technical effects of improving the flight distance accomplished by facilitating turbulent transition by the open cells are not sufficiently obtained. On the other hand, when the depth is larger than 0.5 mm, the original shape of the dimple is not kept. In addition, the core under the cover is exposed, and the appearance of the golf ball is degraded.

[0034] In the golf ball of the present invention, it is desired for the open cell to have an open area of 0.05 to 5.0 mm², preferably 0.13 to 3.1 mm², more preferably 0.0.20 to 2.0 mm², most preferably 0.20 to 1.5 mm². When the open area is smaller than 0.05 mm², the technical effects of improving the flight distance accomplished by facilitating turbulent transition are not sufficiently obtained. On the other hand, when the open area is larger than 5.0 mm², the shape of the dimple placed is not kept, and the original aerodynamic performance of the dimple is not sufficiently obtained.

[0035] In the golf ball of the present invention, it is desired for the open cell to have a number of 200 to 10,000/cm², preferably 200 to 6,000/cm², more preferably 240 to 5,500/cm², most preferably 600 to 5,000/cm². When the number is smaller than 200/cm², the technical effects of improving the flight distance accomplished by facilitating turbulent transition are not sufficiently obtained. On the other hand, when the number is larger than 10,000/cm², the shape of the dimple placed is not kept, and the original aerodynamic performance of the dimple is not sufficiently obtained.

[0036] In the golf ball of the present invention, it is desired for the open cell to have a diameter of 0.4 to 1.7 mm, preferably 0.5 to 1.4 mm. When the diameter is larger than 1.7 mm, the shape of the dimple is not kept, and the desired dimple effect is not sufficiently obtained. On the other hand, when the diameter is smaller than 0.4 mm, the technical effects of improving the flight distance accomplished by facilitating turbulent transition by the open cells are not sufficiently obtained. Since the open cells are formed by foaming, there is a case that the shape thereof is not uniformly and is not true circular. If so, the major axis of the open cell is preferably within the above range.

[0037] It is possible to control the depth, open area (diameter) and number of the open cell by adjusting solidifying condition such as solidifying time, the type and amount of the blowing agent. That is, when solidifying the cover composition with foaming,

[0038] (i) it is possible to increase the diameter of the cell formed by foaming (the diameter and depth of the open cell) and to decrease the number of the cell formed by foaming (the open cell) by increasing the solidifying time, and on the other hand, it is possible to decrease the diameter of the cell formed by foaming (the diameter and depth of the open cell) and to increase the number of the cell formed by foaming (the open cell) by decreasing the solidifying time, and

[0039] (ii) it is possible to increase the diameter of the cell formed by foaming (the diameter and depth of the open cell) by increasing the amount of the blowing agent, and on the other hand, it is possible to decrease the diameter of the cell formed by foaming (the diameter and depth of the open cell) by decreasing the amount of the blowing agent.

[0040] In order to foam the cover used for the golf ball of the present invention, gas, such as nitrogen, carbon dioxide and air, and a blowing agent, which generates gas by heating at a temperature of 50 to 150° C., may be used. Preferred is a blowing agent because of easy handling. In the present invention, the cover composition comprises a base polymer and blowing agent. Examples of the base polymer include thermoplastic resins, such as ionomer resin, thermoplastic elastomer (including polyurethane-based thermoplastic elastomer, polyester-based thermoplastic elastomer, polyamide-based thermoplastic elastomer); thermosetting resins, such as thermosetting polyurethane resin; vulcanized rubber (the same as described for the core); or combinations thereof. It is preferable to comprise ionomer resin and thermoplastic elastomer as a main component, because it is easy to mold with foaming by injection molding and the edge of the open cell is difficultly damaged at the time of hitting and has excellent durability. The wording “comprise as a main component” as used herein refers to be in the amount of not less than 50 parts by weight, based on 100 parts by weight of the base polymer.

[0041] The ionomer resin may be a copolymer of α-olefin and α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms, of which a portion of carboxylic acid groups is neutralized with metal ion, a terpolymer of α-olefin, α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms and α,β-unsaturated carboxylic acid ester, of which a portion of carboxylic acid groups is neutralized with metal ion or mixture thereof. Examples of the α-olefins in the ionomer preferably include ethylene, propylene and the like. Examples of the α,β-unsaturated carboxylic acid in the ionomer include acrylic acid, methacrylic acid, fumaric acid, maleic acid, crotonic acid and the like, preferred are acrylic acid and methacrylic acid. Examples of the α,β-unsaturated carboxylic acid ester in the ionomer include methyl ester, ethyl ester, propyl ester, n-butyl ester and isobutyl ester of acrylic acid, methacrylic acid, fumaric acid, maleic acid, crotonic acid and the like. Preferred are acrylic acid esters and methacrylic acid esters. The metal ion, which neutralizes a portion of carboxylic acid groups of the copolymer or terpolymer, includes an alkali metal ion, such as a sodium ion, a potassium ion, a lithium ion and the like; a divalent metal ion, such as a zinc ion, a calcium ion, a magnesium ion and the like; a trivalent metal ion, such as an aluminum, a neodymium ion and the like; and mixture thereof. Preferred are sodium ions, zinc ions, lithium ions and the like, in view of rebound characteristics, durability and the like.

[0042] The ionomer resin is not limited, but examples thereof will be shown by a trade name thereof. Examples of the ionomer resins, which are commercially available from Du Pont-Mitsui Polychemicals Co., Ltd. include Hi-milan 1555, Hi-milan 1557, Hi-milan 1601, Hi-milan 1605, Hi-milan 1652, Hi-milan 1702, Hi-milan 1705, Hi-milan 1706, Hi-milan 1707, Hi-milan 1855, Hi-milan 1856, Hi-milan AM7316 and the like. Examples of the ionomer resins, which are commercially available from Du Pont Co., include Surlyn 8945, Surlyn 9945, Surlyn 6320, Surlyn 8320 and the like. Examples of the ionomer resins, which are commercially available from Exxon Chemical Co., include Iotek 7010, Iotek 8000 and the like. These ionomer resins may be used alone or in combination.

[0043] Examples of the thermoplastic elastomers, which are commercially available, include polyester-based thermoplastic elastomer, which is commercially available from Toray-Do Pont Co., Ltd. under the trade name of “Hytrel” (such as “Hytrel 3548”, “Hytrel 4047”); polyamide-based thermoplastic elastomer, which is commercially available from Atofina Japan Co., Ltd. under the trade name of “Pebax” (such as “Pebax 2533”); polyurethane-based thermoplastic elastomer, which is commercially available from BASF Japan Co., Ltd. under the trade name of “Elastollan” (such as “Elastollan ET880”); olefin-based thermoplastic elastomer available from Mitsubishi Chemical Co., Ltd. under the trade name “Thermoran” (such as “Thermoran 3981N”); polyolefin-based thermoplastic elastomer, which is commercially available from Sumitomo Chemical Co., Ltd. under the trade name of “Sumitomo TPE” (such as “Sumitomo TPE3682” and “Sumitomo TPE9455”); styrene-based thermoplastic elastomer, which are commercially available from Mitsubishi Chemical Co., Ltd. under the trade name of “Rabalon” (such as “Rabalon SR04”); styrene-based thermoplastic elastomer available from Asahi Kasei corporation under the trade name “Tuftec” (such as “Tuftec H1051”); and the like.

[0044] In the present invention, organic blowing agent or inorganic blowing agent, such as sodium bicarbonate (baking soda) may be used as the blowing agent, and the organic blowing agent is suitable because it has proper rate of gas generation from thermal decomposition thereof and the decomposition temperature can be easily adjusted by using a blowing assistant and the like. Examples of the blowing agents include N,N′-dinitrosopentamethylenetetramine, which is commercially available from Ouchishinko Chemical Industrial Co., Ltd. under the trade name of “Spongepaste No. 4”; azodicarbonamide, which is commercially available from Eiwa Chemical Ind. Co., Ltd. under the trade name of “Vinyfor AC”; azobisisobutylonitril, which is commercially available from Eiwa Chemical Ind. Co., Ltd. under the trade name of “Vinyfor AZ”; 20% compound of barium azodicarboxylate, which is commercially available from Eiwa Chemical Ind. Co., Ltd. under the trade name of “Polythlene ES#201”; benzenesulfonylhydrazide, which is commercially available from Bayer AG; p,p′-oxybis(benzenesulfonylhydrazide), which is commercially available from Eiwa Chemical Ind. Co., Ltd. under the trade name of “Neocellborn N#1000”, toluenesulfonylhydrazide, which is commercially available from Eiwa Chemical Ind. Co., Ltd. under the trade name of “Unifor H”; toluenesulfonylhydrazide derivatives, which is commercially available from Eiwa Chemical Ind. Co., Ltd. under the trade name of “Unifor NH#500”, “Unifor NH#800”, “Unifor NH#1000”; p-toluenesulfonylsemicarbazide, which is commercially available from Uniroyal Chem. Co.; and the like. These blowing agents may be used alone or in combination. The amount of the blowing agent is from 0.3 to 3.0 parts by weight, preferably from 0.5 to 2.0 parts by weight, more preferably from 0.8 to 1.5 parts by weight, based on 100 parts by weight of the base polymer for the cover. When the amount of the blowing agent is smaller than 0.3 parts by weight, the foaming is not sufficiently conducted. On the other hand, when the amount of the blowing agent is larger than 3.0 parts by weight, the foaming is severely conducted, and the desired open cell is not obtained.

[0045] The blowing agent may be used with a blowing assistant, such as salicylic acid, zinc oxide, zinc stearate, urea compounds (for example, which is commercially available from Eiwa Chemical Ind. Co., Ltd. under the trade name of “Cellpaste A”) and the like. If used, the amount of the blowing assistant is preferably the same as that of the blowing agent (the same range as the blowing agent). That is, the amount of the blowing assistant is from 0.3 to 3.0 parts by weight, preferably from 0.5 to 2.0 parts by weight, more preferably from 0.8 to 1.5 parts by weight, based on 100 parts by weight of the base polymer for the cover. When the amount of the blowing assistant is smaller than 0.3 parts by weight, it is difficult to adjust the decomposition temperature. On the other hand, even if the amount of the blowing assistant is larger than 3.0 parts by weight, the technical effect of adjusting the decomposition temperature is not improved.

[0046] The cover of the present invention may be formed by conventional methods, which have been known in the art and used for forming the cover of the golf balls, and the method is not specifically limited. For example, there can be used a method comprising molding the cover composition into a foamed semi-spherical half-shell in advance, covering the core with the two half-shells, followed by press molding, or a method comprising injection molding the cover composition directly on the core, which is covered with the cover, to cover it.

[0047] In the golf ball of the present invention, the diameter and number of the cells formed by foaming are adjusted by controlling the molding temperature, solidifying time (cooling time), the amount of the blowing agent and the like in the above conventional method to obtain the desired diameter, depth and number of the open cells. The molding temperature, which varies depending on the type and amount of the blowing agent, is desirably within the range of 130 to 250° C., preferably 150 to 230° C., more preferably 160 to 220° C. When the molding temperature is lower than 130° C., inner strain remains in the molded article, and it is a reason for the deformation and crack of the resulting golf ball. On the other hand, when the molding temperature is higher than 250° C., a sink mark is caused in the cover.

[0048] The solidifying time, which is cooling time until the molded article is solidified, varies depending to the molding temperature, but it is desired to be within the range of 0.5 to 3 minutes, preferably 0.7 to 2 minutes, more preferably 0.9 to 1 minute. When the solidifying time is shorter than 0.5 minutes, the foaming is not sufficiently obtained. On the other hand, when the solidifying time is longer than 3 minutes, the molding time (cost) is increased.

[0049] The cover composition used in the present invention may optionally contain nucleating agent (such as barium sulfate, calcium carbide), pigments (such as titanium dioxide, etc.) and the other additives such as a dispersant, an antioxidant, a UV absorber, a photostabilizer and a fluorescent agent or a fluorescent brightener, etc., in addition to the base polymer as a main component and blowing agent as long as the addition of the additives does not deteriorate the desired performance of the golf ball cover. If used, the amount of the pigment is preferably 0.1 to 5.0 parts by weight, based on 100 parts by weight of the base polymer for the cover.

[0050] At the time of molding the cover, many depressions called “dimples” are formed on the surface of the golf ball. In the golf ball of the present invention, since the cover has open cells formed by foaming on the surface thereof, it is possible to solve the problem that the dimple edge is not sharp in case of forming the open cells by blasting treatment, which reduces the original aerodynamical effects accomplished by the original dimples. In the golf ball of the present invention, furthermore, pretreatment for painting and paint may be optionally provided after the cover is molded. However, the open cells are filled up with the paint, the technical effects accomplished by the presence of the open cells are not sufficiently obtained, and the desired performance of the resulting golf ball is not sufficiently obtained. Therefore, it is desired for the paint to have a thickness of 4 to 16 μm, preferably 7 to 13 μm.

[0051] Examples of the pretreatment, typically, include grinding or blasting treatment, washing treatment, flaming treatment and the like. In the golf ball of the present invention, preferred is the grinding treatment, because the foamed cells near the surface can be exposed to form the open cells by the treatment. Therefore, in the case, the open cells can be formed by the grinding treatment as the pretreatment for painting without increasing further processing step to expose the foamed cells near the surface. The grinding treatment is a method of grinding the surface of the golf ball body by using abrasive stone or abrasive cloth. Particularly preferred is barrel grinding treatment (such as roll flow barrel grinding), because the productivity is excellent and it is possible to uniformly grind throughout the surface of the golf ball body, or it is always possible to uniformly grind between the golf balls. The barrel grinding treatment is a method of grinding by rotating a barrel apparatus, in which the golf ball and abrasive stone are placed.

[0052] The golf ball of the present invention is formed to a diameter of 40 to 45 mm, particularly 42 to 44 mm. In order to reduce air resistance within the range according to the USGA (United States Golf Association) rule, golf ball of the present invention is formed to a diameter of at least 42.67 mm (preferably 42.67 to 42.82 mm) and a weight of not more than 45.93 g.

EXAMPLES

[0053] The following Examples and Comparative Examples further illustrate the present invention in detail but are not to be construed to limit the scope of the present invention.

[0054] Production of core

[0055] The rubber composition for the core having the formulation shown in Table 1 was mixed, and then vulcanized by press-molding in the mold at 165° C. for 20 minutes to obtain spherical solid core having a diameter of 38.7 mm.

[0056] Table 1 TABLE 1 Amount Core composition (parts by weight) Polybutadiene rubber *1 100 Zinc acrylate 25 Zinc oxide 5 Dicumyl peroxide 1

EXAMPLES 1 TO 6 AND COMPARATIVE EXAMPLE 1

[0057] The cover compositions having the formulations shown in Table 2 were foamed and solidified on the resulting core by injection molding using a mold having dimples at the molding temperature and solidifying time shown in Tables 3 and 4 to form a cover layer having a thickness of 1.6 mm and obtain golf balls having a diameter of 42.7 mm. With respect to the resulting golf balls, the diameter, depth, open area and number of the open cell; elevation angle of trajectory; and the flight distance were measured, and the results are shown in Tables 3 and 4.

EXAMPLE 7 AND COMPARATIVE EXAMPLE 2

[0058] The golf balls having a diameter of 42.7 mm were obtained in the same way as described in Examples 1 to 6 and Comparative Example 1, except that urethane paint was applied on the surface in a thickness of 10 μm after forming the cover layer. With respect to the resulting golf balls, the diameter, depth, open area and number of the open cell; elevation angle of trajectory; and the flight distance were measured, and the results are shown in Table 4.

Comparative Examples 3

[0059] The golf ball was obtained in the same way as described in Comparative Example 1, except that the surface of the golf ball was treated by blasting treatment. The treatment was conducted by blowing iron powder together with compressed air to strike the iron powder against the surface of the golf ball. With respect to the resulting golf ball, the diameter, depth, open area and number of the open cell; elevation angle of trajectory; and the flight distance were measured, and the results are shown in Table 4. The test methods are described later. TABLE 2 (parts by weight) Cover composition a b c Hi-milan 1605 *2 50 50 50 Hi-milan 1706 *3 50 50 — Surlyn 9945 *4 — — 50 Azodicarbonamide  1 —  1

[0060] (Test methods)

[0061] Flight performance

[0062] After a No. 5 iron club (I#5; “XXIO” loft angle=26 degrees, S shaft, manufactured by Sumitomo Rubber Industries, Ltd.) was mounted to a swing robot manufactured by Golf Laboratory Co and a golf ball was hit at head speed of 41 m/sec, the elevation angle of trajectory and flight distance were measured. As the flight distance, total that is a distance to the stop point of the hit golf ball was measured. The elevation angle of trajectory was determined by measuring an angle made with the horizontal by a straight line, which passes through the peak of the trajectory and an observation point at hitting point. The measurement was conducted 20 times (n=20) for each golf ball, and the average is shown as the result of the golf ball.

[0063] The dispersion of the flight distance was determined by calculating the difference between the minimum value and the maximum value thereof. The distance from the dropping point to the objective direction of trajectory of the hit golf ball was measured. The directivity was determined by calculating the total of the maximum value of the distance on the left of the objective direction and that of the distance on the right of the objective direction. The smaller the total value is, the more excellent the directivity is.

[0064] (Test results) TABLE 3 Example No. 1 2 3 4 5 (Cover) Composition a a a a a Molding condition Molding 200 200 200 200 200 temperature(° C.) Solidification 0.8 1.0 1.5 2.0 0.5 time (min) Foaming foamed foamed foamed foamed foamed Open cell Diameter (mm) 0.4 0.5 1.4 2.0 0.1 Depth (mm) 0.05 0.07 0.20 0.5 0.01 Area (mm²) 0.13 0.20 1.5 3.1 0.008 Number (/cm²) 5500 4000 4900 240 90000 Elevation 15.7 15.5 15.2 14.9 15.8 angle of trajectory (degree) Flight distance (m) 191.6 193.2 193.6 192.4 191.1 Dispersion of flight 10 11 9 10 9 distance (m) Directivity (m) 5 4 6 6 5

[0065] TABLE 4 Comparative Example No. Example No. 6 7 1 2 3 (Cover) Composition a c b b a Molding condition Molding 200 200 200 200 200 temperature(° C.) Solidification 3.0 1.2 0.5 3.0 0.3 time (min) Foaming foamed foamed No No No Open cell Diameter (mm) 3.0 0.5 — — 0.3 Depth (mm) 0.7 0.07 — — 0.05 Area (mm²) 7.1 0.20 — — 0.07 Number (/cm²) 110 4000 — — 10000 Elevation angle of 14.8 15.6 15.9 15.9 15.7 trajectory (degree) Flight distance (m) 190.9 192.9 190.5 190.4 191.1 Dispersion of 10 9 9 10 16 flight distance (m) Directivity (m) 6 7 5 7 11

[0066] As is apparent from the results of Tables 3 to 4, in the golf balls of the present invention of Examples 1 to 7, when compared with the golf balls of Comparative Examples 1 to 2, the flight distance is long, and the flight performance is excellent. In the golf balls of Examples 5 and 6, which are within the scope of the present invention, the flight distance is superior to the golf balls of Comparative Examples, but is slightly shorter than the other Examples, because the golf ball of Example 5 has small open area of the open cell and the golf ball of Example 6 has large depth and small area of the open cell and small number of the open cell.

[0067] On the other hand, in the golf ball of Comparative Example 1 (without applying paint) and the golf ball of Comparative Example 2 (applying paint), since the foamed cover is not used and the cover has no open cells, the flight distance is very short. In the golf ball of Comparative Example 3, since the open cells are formed by blasting treatment, the flight distance is very short. 

What is claimed is:
 1. A golf ball comprising a core and a cover covering the core, wherein the cover has many fine open cells formed by foaming on the surface thereof.
 2. The golf ball according to claim 1, wherein the open cell has a depth of 0.05 to 0.5 mm, an opening area of 0.005 to 5.0 mm² and a number of 200 to 10,000 per 1 cm².
 3. A method of making a golf ball comprising a core and a cover covering the core comprising the steps of (a) forming the core, (b) providing a foamable cover composition, and (c) covering the cover formed from the cover composition on the core to form the golf ball, wherein the cover has many fine open cells formed by foaming on the surface thereof.
 4. The method according to claim 3 further comprising the step of (d) removing the surface of the cover to form many fine open cells on the surface of the golf ball, after the step (c). 